Measuring the Thermal Conductivity of Buried Sub-Surface Materials and Substrates

SSTR offers the penetration ability needed to measure thermal conductivities for structures buried too deep for traditional methods.

The Challenge

Measuring the thermal conductivity of sub-surface buried substrates is of significant practical interests. However, this remains challenging with traditional pump–probe spectroscopies due to their limited thermal penetration depths.

The Solution

Because SSTR operates in a steady state regime, the measurement volume over which SSTR measures the thermal conductivity can penetrate under the surface to depths as large as the laser spot size.  This capability allows SSTR to measure the thermal conductivity of materials beneath the surface based on tuning the pump and probe laser spot size.

Here, we demonstrate this by measuring the thermal conductivity of substrates that are buried 100 nm to a few microns under the surface below dielectric films.  Specifically, here we generate measurements for SiO2, GaN substrates under 2.05 microns of GaN films, and sapphire substrates under 2 microns of AlN.

For estimating the uncertainty of SSTR measurements of a buried substrate a priori, sensitivity calculations provide the best means. Thus, detailed sensitivity calculations are provided to guide future measurements. Due to the steady-state nature of SSTR, it can measure the thermal conductivity of buried substrates that are traditionally challenging by transient pump–probe techniques, exemplified by measuring three control samples.

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